Arctic research Initiative funded project: Is thawing permafrost as a result of global warming a possible significant source of degradable carbon for microbiota residing in situ and in Arctic rivers?

Northern high-latitude ecosystems contain about half of the world’s soil carbon, most of which is stored in permanently frozen soil (permafrost). Global warming through the 21st century is expected to induce permafrost thaw, which will increase microbial organic matter (OM) decomposition and release large amounts of the greenhouse gasses methane and carbon dioxideinto the atmosphere. In addition, Arctic rivers are a globally important source of terrestrial organic carbon to the ocean and further permafrost melting will impact surface runoff, directly affecting groundwater storage and river discharge. Up to now, it remains largely unknown to what extent the ancient OM stored in newly thawing permafrost can be consumed by microbes in situ or by microbes residing in Arctic rivers which become exposed to newly discharged permafrost OM. In addition, we know little about which microbes are capable of degrading permafrost OM.

During a field trip to the Toolik Lake Arctic Long Term Ecological Research (LTER) field station in northern Alaska in August 2008, we cored permafrost located near the Kuparuk River down to 110 cm below the active layer (i.e. the top layer which melts each summer) and analyzed the initial microbial enzymatic cleavage of particulate OM (POM) stored in permafrost. Alkaline phosphatase activity remained fairly constant throughout the permafrost and was only one order of magnitude lower than in the active layer. The latter enzyme cleaves organic phosphoesters into phosphate, which could cause eutrophication of lakes and rivers via ground water discharge. Similar results were found for beta-glucosidase, which cleaves cellobiose into glucose. This process could fuel heterotrophic bacteria to produce carbon dioxide which, in return, could be converted to the stronger greenhouse gas methane by methanogenic archaea. Leucine aminopeptidase activities, on the other hand, were highest in the top Sphagnum root layer and quickly dropped to below detection limit below the active layer.In addition, time series experiments showed that alkaline phosphatase and beta-glucosidase was induced in bacteria from the Kuparuk River (both pelagic and benthic), when exposed to OM from sterilized permafrost, to the same extent than when fuelled with OM derived from sterilized active layer.

Overall, we showed that Pleistocene permafrost harbors a significant amount of labile OM. In the near future, microbes involved in the enzymatic degradation of permafrost POM will be identified using Enzyme Linked Fluorescence-Fluorescence In Situ Hybridization (ELF-FISH).